Even machines that weren't designed to be mobile can make use of a wireless NIC - it beats stringing wire around the house to meet each PC.
It's a good question - I expect it'll be a matter of time before WiFi becomes common on at least higher-end motherboards.
In the meantime, if you don't have any free PCI slots there is always the possibility of using a USB WiFi NIC.
The first time I built a PC (it was a screamingly fast Pentium 200Mhz with MMX! - all the way back in the distant past of 1997) the motherboard didn't have built-in anything really - I had to buy a separate video and sound card, an external modem (with K56 Flex "highspeed"), and a network adapter (10baseT because the 100baseT cards were too expensive). I think the board had a couple of PCI slots, but mostly it had ISA.
Man, it ran Windows 95SE SO FAST. It was a monster at the time.
Now my processor is running at 11x the clock speed/frequency, and the actual performance is many times more than that.
Heck, even the "new" AGP slot is on its way out now. It's hard to find new motherboards that don't have PCI-Express for video.
So it's just a matter of time before WiFi becomes standard on motherboards. And also before modems no longer come standard (my new MSI mATX board does NOT come with a modem built-in - but I had to check just now before I realised that!).